1. Field of the Invention
The present invention relates to novel polyarylate resins end-capped with nadimide groups and a method for preparing them.
The present invention also relates to novel thermoset type of polyarylate resins having excellent mechanical properties as well as chemical resistance and high-temperature resistance, and a method for preparing them.
2. Description of the Prior Art
Polyarylate resins are, in general, wholly aromatic polyester resins which can be prepared by the polycondensation of aromatic dicarboxylic acid with aromatic diol. The molecular structures of the polyarylate resins vary depending on the monomers employed But typical examples thereof are those represented by the following formula(i) ##STR2## prepared by the polycondensation of bisphenol A as a diol component with terephthalic acid or isophthalic acid as an aromatic dicarboxylic acid component.
The polyarylate resins of the formula(i) are disclosed in Japanese Patent Laid-Open Nos.49-101,944 and 55-115,871. They are transparent and noncrystalline resins which have the following properties:
(1) They have improved high-temperature resistance and excellent mechanical properties without being reinforced by glass fibers owing to high density of aromatic benzene rings in their molecule;
(2) there is a sufficient gap between their softening point and thermal decomposition temperature, so that any of extrusion molding, hollow molding and injection molding may be used;
(3) they have a limiting oxygen index of 37%, so that the resins by themselves possess good flame retardant property;
(4) they have high ultraviolet absorbance to be good in weather-resistance; and
(5) they have good compatibility with other resins and thus formulations are easily made, so that the physical properties can be modified in a wide range.
Thus, the polyarylate resins represented by the formula(i) are a sort of specific engineering plastics which have good physical properties. They have been used as a variety of structural and packaging materials for, for example, switches, sockets, microwave ovens, relay cases or substrates in the electric and electronic industries, interior and exterior components of clocks, components of optical instrument, gas cutting apparatus or pump housings in the mechanical industry, and lenses or gauge panels in the auto industry.
Most polyarylate resins presently available in the market are those of the formula(i). The existing resins of the formula(i) have improved high-temperature resistance, but they require a melt processing at a temperature as high as 400.degree. C. due to the high melt viscosity. Such processing at the high temperature leads to poor processability and significant thermal decomposition of the resins, and requires specific process equipments causing higher process cost.
To eliminate the disadvantages of the conventional polyarylate resins, U.S. Pat. No.4,126,602 suggested the use of resins wherein a part of the terephthaloyl chloride and isophthaloyl chloride moieties are replaced by aliphatic dicarboxylic acid and U.K.Patent No.2,085,458 suggested the use of resins wherein dicarboxylic acid having a flexible group such as --O--, --S--, --SO--, --SO.sub.2 -- and --CO-- is present between two benzene rings. Further, Japanese Patent Laid-Open Nos. 51-136793 and 56-29684 suggested that, instead of using bisphenol A, biphenols wherein a bond such as --O--, --S--, --SO--, etc. is present between two benzene rings or halogen-substituted or side chain-substituted bisphenol A(or biphenol) are used. The polyarylate resins modified as described above have a decreased melt molding temperature owing to the increased chain flexibility, but their high-temperature resistance, mechanical properties and weather resistance are significantly decreased Therefore the resins are impractical.
Examples of the methods for preparing polyarylate resins generally include the solution polymerization, melt polymerization and interface polymerization methods.
The solution polymerization method comprises converting aromatic dicarboxylic acid to the acid chloride form and then reacting the latter with aromatic diol in the presence of triethylamine or pyridine. For example, terephthalic acid or isophthalic acid are converted to their acid chloride form, i.e., terephthaloyl chloride or isophthaloyl chloride, respectively. This method is classified into "low" and "high" temperature polymerization methods depending on the solvents and the reaction temperatures used. The typical example of the low temperature polymerization method comprises the polycondensation at temperatures of -10.degree. C. to 30.degree. C. using tetrahydrofuran as a solvent as disclosed in U.S. Pat. Nos. 3,234,168; 4,049,629 and 4,051,107. A typical example of the high temperature polymerization comprises polycondensation at elevated temperatures of 215 .degree. C. to 220.degree. C. using a higher boiling solvent such as dichloroethylbenzene or ditolylmethane as disclosed in U.S. Pat. No. 3,133,898; 3,702,838 and 3,733,306. However, both methods have disadvantages in that it is difficult to give polymers having high molecular weights and the cost for the solvent recovery and purification is too high to have economical merit on an industrial scale.
The melt polymerization method comprises the polycondensation of a diacetate compound of aromatic diol with aromatic dicarboxylic acid or of a diphenyl ester compound of aromatic dicarboxylic acid with aromatic diol at elevated temperatures of 220.degree. C. to 330.degree. C. in the presence of a catalyst as disclosed in Japanese Patent Laid-Open No. 53-35796, U.S. Pat. Nos. 3,317,464; 3,553,167 and 3,975,487. This polymerization method has problems in that the melt viscosity of the reaction rapidly increases as the polymerization reaction proceeds, and thereby it is difficult to give polymers having high molecular weights. Furthermore, since the polymerization reaction is carried out at a higher temperature, thermal decompositions of the polymers or side reactions may concur with the polymerization reaction causing the polymers to have unsatisfactory color.
Finally, the interface polymerization method comprises dissolving acid chloride of aromatic dicarboxylic acid in an organic solvent which cannot be mixed with water such as dichloromethane and adding a solution of aromatic diol in an aqueous solution of sodium hydroxide into the obtained mixture with violent stirring. If necessary, the phase transfer catalyst such as trimethyl ammonium bromide may be used. See S.C. Temin, "Interfacial Synthesis" ed. F. Mellich and C.E. Carraher, Dekker, New York, 1977, Vol. II, p27; H.-B. Tsai and Y.-D. Lee, J. Polym. Sci., Polym. Chem. Ed. 1987, 25, 1505; U.S. Pat. No. 4,229,565. This polymerization method is advantageous in that the polymerization is carried out even at room temperature, the purity of the raw materials does not matter and the equivalent ratio between the reactants is not necessarily required.
The active researches have been devoted to improving the processability of polyarylate resins since the 1980's. The typical examples thereof are directed to introducing flexible groups into the main chains of polyarylate resins as reported in P. Bajaj, D.N. Khanna and G.N. Babu, Eur. Polym. J., 15, 1083(1979), P. Bajaj and D.N. Khanna, Eur. Polym. J., 17, 275(1981), D.N. Khanna, P. Bajaj and A.K. Gupta, Polymer, 24,596(1983) and to preparing polyarylate resins wherein t-butyl side chains are present as reported in M.-A. Kakimoto, S. Harada, Y. Oishi, Y. Imai, J. Polym. Sci., Polym. Chem. Ed., 25, 2747(1987). But the above methods result in the deterioration of physical properties of polyarylate resins, although solubility and oxidation resistance of the obtained resins are improved.
Further, recent researches made by the present inventors have provided new polyarylate resins where silicon-containing side chains are introduced and which have a good balance of high-temperature resistance and physical properties[See Kil-Yeong Choi, Mi Hie Yi and Sam-Kwon Choi, J. Polym. Sci., Polym. Chem. Ed., in press].
In addition, the assiduous studies have been made for years to improve processability and chemical resistance of polyarylate resins without significant deterioration of their physical properties and high-temperature resistance. As results of these studies, certain high-temperature resistant polymers having new structures have been produced. One of the approaches is to synthesize such polymers as polyetheretherketone or sulfone block copolymer with improved chemical resistance by providing crystallinity on the main chain of the polymer. Another approach is to synthesize heat-resistant polymers with good chemical resistance by introducing curable, unsaturated reactive groups such as a maleimide group of the formula(ii), cyanato group of the formula(iii), nadimide group of the formula(iv) or acetylene group of the formula(v) into the ends of thermoplastic resinous oligomers. ##STR3## As a typical example thereof, C.H. Sheppard(1980) has proposed the method of end-capping polysulfone(MW=20000)(which is one of the thermoplastic resins) with nadimide groups followed by thermal curing to give the end-capped polysulfone of the formula(vi) with good chemical resistance. ##STR4##
Furthermore, the synthesis and physical properties of acetylene-terminated polysulfones(ATS) of the formula(vii) were studied and reported by P.M. Hergenrother of NASA(1984). ##STR5##
However, the above-described studies have been devoted to some thermoplastic resins such as polyethersulfone, polysulfone, etc., and little attention was given to the end-capped polyarylate resins except for the research made by P.M. Hergenrother. And the research for the end-capping reaction and curing mechanisms of various oligoester were reported only recently by J.DE Abajo.
Based on the result of the above-mentioned studies, the present inventors have made extensive studies using as a basic resin a polyarylate resin which is one of the engineering plastics in order to improve processability and chemical resistance, said properties having caused many problems in conventional polyarylate resins. As a result, the present inventors have now found that by introducing nadimide groups into conventional polyarylate resins and then curing them thermally, heat-resistant, cured polyarylate resins can be produced which are intended for use as structural materials at high temperatures in, for example, auto, electric and electronic, aerospace industries. Based on these findings, we have completed the present invention.